1. Field of the Invention
The present invention pertains to a fluid pump control system for excavators and, more specifically, to a fluid pump control system adapted for controlling a discharge quantity of a variable displacement fluid pump in proportion to a manipulation amount of a remote control valve.
2. Description of the Related Art
Known systems for controlling a fluid pump in excavators include a positive pump control system that controls the discharge quantity of a pump in proportion to the magnitude of an input signal and a negative pump control system that controls the pump discharge quantity in inverse proportion to the magnitude of the input signal.
One example of the positive control system is shown in FIG. 1. As shown, the positive control system comprises variable displacement fluid pumps 101, 102 for producing hydraulic flows into main fluid pressure lines 110, 111 along which a plurality of spools 103A, 103B, 104A, 104B of control valves 103, 104 are disposed one after another. The spools 103A, 103B, 104A, 104B are also in fluid communication with the fluid pumps 101, 102 through parallel lines.
Remote control valves 105, 106 are provided to reduce the pressure of a hydraulic flow generated by a pilot pump (not shown) to thereby create pilot signal pressures which in turn are transmitted through pilot signal lines 105A-D, 106A-D to pressure receiving parts on opposite sides of the spools 103A, 103B, 104A, 104B of the control valves 103, 104. Responsive to the pilot signal pressures, the spools 103A, 103B, 104A, 104B are shifted in one direction to allow the hydraulic flows of the fluid pumps 101, 102 to be supplied to a variety of actuators not shown in the drawings.
Operatively connected to swash plates of the variable displacement fluid pumps 101, 102 are discharge quantity regulators 101A, 101B that remain in fluid communication with shuttle valves 107A, 107B, 108A, 108B for selecting the greatest one of the pilot signal pressures outputted from the remote control valves 105, 106 to supply a quantity control signal pressure Pi to the regulators 101A, 101B. In proportion to the magnitude of the quantity control signal pressure Pi, the regulators 101A, 101B serve to variably control the discharge quantity of the fluid pumps 101, 102.
Referring to FIG. 2, which graphically illustrates the correlation of the quantity control signal pressure Pi and the discharge quantity Q of the fluid pumps 101, 102, it can be seen that the discharge quantity Q of the fluid pumps 101, 102 is increased from Q1 to Q2 as the remote control valves 105, 106 generate the pilot signal pressures of greater magnitude and hence the quantity control signal pressure Pi supplied by the shuttle valves 107A, 107B, 108A, 108B grows from P1 to P1. Inversely, reduction of the quantity control signal pressure Pi results in proportional decrease of the discharge quantity Q of the fluid pumps 101, 102.
In the positive pump control system as noted above, if the remote control valves 105, 106 are manipulated simultaneously and generate a couple of pilot signal pressures P1, P2 for the purpose of causing the excavator to perform composite operations through the simultaneous actuation of at least two hydraulic actuators, the shuttle valves 107A, 107B, 108A, 108B adopt the greater one P2 of the pilot signal pressures P1, P2 as the quantity control signal pressure Pi but discard the smaller one P1.
As a result, the fluid pumps 101, 102 produce the hydraulic flows of the discharge quantity Q2 corresponds to the adopted pilot signal pressure P2, which means that the quantity of the hydraulic flows is not sufficient to actuate two or more actuators at one time and carry out the composite operations smoothly.
In contrast, the negative pump control system can acquire a quantity control signal pressure that covers the entire pilot pressures applied to the respective spools of the control valve, thus removing the drawbacks inherent in the positive pump control system. In the negative pump control system, however, an orifice and a relief valve are attached to the downstream-most side of a bypass line in order to detect the quantity control signal pressure. The orifice and the relief valve tend to create a pressure loss which makes it difficult to accurately detect the quantity control signal pressure. This results in the fluid pumps discharging an inaccurately controlled quantity of hydraulic flows, which may cause a difficulty in performing the composite operations in a precise manner.